Mechanical stability limits of bi-layer thermal barrier coatings

Thermal barrier coatings based on yttria stabilized zirconia (YSZ) have reached a state in development, where a further increase in lifetime and/or maximum operating temperature has become more and more difficult. Nevertheless, the demand for even higher operating temperatures is persisting, and it is pushing the YSZ-based thermal barrier coatings to and beyond their limit. Especially phase instabilities of YSZ at temperatures above 1200°C lead to a fast deterioration of the ceramic top coating. Consequently, the search for new materials with low thermal conductivity, relatively low coefficient of thermal expansion and phase stability at temperatures well above 1200°C was started, to replace YSZ as the top layer material [1;2]. Among the candidates, gadolinium zirconate (Gd2Zr2O7, GZO) and lanthanum zirconate (La2Zr2O7, LZO) have proven to be viable choices; however, the fracture toughness of these materials is considerably lower than that of YSZ which results in a lower strain tolerance and early failure of single-layered coatings [3]. It is therefore advantageous to create bi-layered coatings, where a top-layer of GZO or LZO is sprayed on a layer of standard YSZ material. As a result, the lower part of the TBC remains a state-of-the-art TBC system and can benefit from the experience with YSZ-TBCs gained so far. The mechanical aspects of such bi-layered ceramic structures have up to now not been investigated in detail. Especially the maximum tolerable load or the corresponding failure strain of the thermal barrier coating is of great interest. This is primarily to ensure that the TBC will not fail during normal operation of the plant. In this work 4-point bend testing with in-situ acoustic emission measurement was used to determine the critical strain to failure of APS bi-layer TBCs based on GZO/YSZ-ceramic. Isothermal oxidation of the specimens was performed at 1050°C and 1100°C, respectively. The use of the acoustic emission technology enables the distinction of individual failure modes in the bi-layer coating. As an example, figure 1 shows the stress-strain-curve and the acoustic emission signal of a bi-layer TBC after isothermal oxidation for 100h at 1050°C. As can be seen from the figure, the top GZO-layer fails prior to failure of the bottom YSZ-layer. The critical strain values are also used to establish a fracture mechanics-based lifetime model. The model delineates areas of safe operation from areas where failure of the TBC system is imminent. Furthermore it can be distinguished between failure of the GZO-layer or the YSZ-layer